Ever since the first integrated circuits it has been desirable to reduce their size so that they may include more complex circuitry without taking up more space. Reduced size integrated circuits generally use less power, cost less and can provide faster performance. Additionally, reduced sized integrated circuits require smaller encasements and can fit more easily into other devices, for example to add GPS functionality to a smartphone or a wristwatch.
An integrated circuit may include one or more main chips and have smaller chips and/or additional elements serving as an electrical interface for the main chips. The footprint of the integrated circuit may be reduced by stacking up multiple layers, for example the main chip on one layer and the rest of the elements on another layer. This introduces the problem of communicating electrical signals between the layers especially if they are on separate substrates (e.g. printed circuit boards), thus requiring wiring between the layers.
In some methods circuit elements may be attached to the bottom side of a substrate inside the integrated circuit. Such an attachment would then require an additional substrate below it with appropriate wiring and supports if the integrated circuit is to be surface mounted onto external circuit boards.